High-resolution, active reflector radio frequency ranging system

ABSTRACT

A radio frequency ranging system is grounded in establishing and maintaining phase and frequency coherency of signals received by a slave unit from a master unit and retransmitted to the master unit by the slave unit. For a preferred embodiment of the invention, coherency is established through the use of a delta-sigma phase-lock loop, and maintained through the use, on both master and slave units, of thermally-insulated reference oscillators, which are highly stable over the short periods of time during which communications occur. A phase relationship counter is employed to keep track of the fractional time frames of the phase-lock loop as a function of the reference oscillator, thereby providing absolute phase information for an incoming burst on any channel, thereby enabling the system to almost instantaneously establish or reestablish the phase relationship of the local oscillator so that it synchronized with the reference oscillator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/771,830, filed on Apr. 30, 2010, which claims the benefit of U.S.Provisional Application No. 61/174,433, filed on Apr. 30, 2009, whichapplications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to active reflector radio frequencydistance measuring systems and, more particularly, to systems whichoperate in a frequency-hopping mode, and employ both an adaptive loopfor oscillator synchronization within a reflector unit in response torandom phase sampling and, optionally, a phase-coherent delta sigmaphase lock loop for accurate signal phase detection and creation atmultiple frequencies.

2. Background and Related Art

Ranging, or the measurement of distance, through the use of phasemeasurements on radio frequency signals transmitted between two pointsin space is a well-known method of determining distance between twopoints. Given that simultaneous transmission and reception on the samecarrier frequency is not possible because of mutual interference,transmission of radio frequency signals on path AB between stations Aand B requires the use of a different carrier frequency for the returnon the path BA.

Thus, such a distance measuring system requires two simultaneouslyoccupied transmission channels and transmitters and receivers on twodifferent frequencies. Such two frequency systems make inefficient useof the available ratio frequency spectrum. In addition, noise on eitherfrequency may interfere with the process.

Ranging effected by measuring the phase of signals sent from point A, topoint B, and back to point A is a well-established technology. Forexample. U.S. Pat. No. 3,243,812 to Williams discloses a system of phasemeasurement for determining distance. U.S. Pat. No. 4,170,773 toFitzsimmons, et al. also discloses a method for determining distance bycomparing the phase of a transmitted signal with one transponded by adistant device.

The Williams method employs the following steps: transmitting amodulated carrier at frequency f₁ from point A (the interrogator) topoint B (the transponder); coherently recovering the modulation (calledrange tones) by means of a receiver at point B; impressing thismodulation on another carrier of frequency f₂, which is then transmittedfrom point B to point A, where the modulation is recovered by a receiverat point A. Two versions of the range tones are then simultaneouslyavailable at point A: the original tone transmitted to point B; and thetone received from point B. The range, or distance, from point A topoint B is determined by measuring the relative phase of the transmittedtones relative to the received tones and computing the distance usingthe following equation:D=cφ/2ω_(m) −dwhere

-   -   D is the distance from point A to point B,    -   c is the velocity of light,    -   φ is the measured phase in radians,    -   ω_(m) is the angular frequency of the modulation in        radians/second,    -   d is the effective distance of the delay through the        transmitting and receiving hardware, and    -   the integer, 2, in the denominator of the equation takes into        consideration the transponded, double-distance path.

Because phase measurements are ambiguous for modulus 2π, thecorresponding distance will be ambiguous for modulus cπ/ω_(m). However,ambiguity can be resolved by taking measurements on multiple tones ateither lower frequencies or at low difference frequencies.

Such prior art systems require transmission and reception simultaneouslyon two different frequencies. Phase measurements are made only at theinterrogator using a single phase reference source, and the interrogatorand receiver hardware perform different functions. The interrogatorcontains the source of the signal sent around the loop from interrogatorto transponder and back, as well as the measurement apparatus fordetermining the relative phase between the transmitted signal and thereceived signal. The transponder functions merely to receive the rangingsignal and to retransmit it with minimum, but known, delay or delayvariation.

One limitation on the use of such prior art ranging systems is therequirement for transmission and reception to occur simultaneously atboth stations, thus requiring clear channel operation on two differenttransmit frequencies at the same time. Moreover, the range measuringcircuits in such systems are started and stopped by reference markerclock signals which are transmitted from each station. The rangecalculation is dependent upon the reference clocks being synchronized orlocked to each other, and significant range errors will be produced ifthe clocks are not maintained in close synchronism.

BRIEF SUMMARY OF THE INVENTION

Embodiments relate to ranging systems, including integrated rangingsystems, which are capable of providing precise measurements withminimal bandwidth utilization.

Embodiments provide an active-reflector, or transponder-type radiofrequency ranging system in which phase and frequency coherency betweenmaster and slave units can be precisely established during periods whenmeasurement data is generated.

Embodiments enable discontinuous transmissions on multiple frequenciesin order to optimize the use of available bandwidth, and to avoidchannels which are either being used for unrelated transmissions orbeset with noise.

Embodiments provide a system of vernier measurement, whereby distancesare measures in terms of an integer number of wavelengths plus afraction of a wavelength that is determined by phase angle differencesbetween two transmissions at different frequencies.

Embodiments eliminate multi-path data from ranging calculations.

A high-resolution active reflector radio frequency ranging systemincludes at least two radio frequency transceivers. One of thetransceivers, acting as a master unit, transmits a radio frequencysignal burst to at least one other designated transceiver which acts asa slave unit and active reflector. The slave unit, actively matches thephase and frequency of the incoming signal and retransmits a signal atthe matched phase and frequency. The slave can retain the phase andfrequency data that it receives for some time before retransmitting thesignal to the master. Within a network, master and slave designationsare arbitrary, as those roles can be temporarily assigned as required.In fact, any unit that initiates a ranging operation is, by definition,a master unit. Each transceiver unit, or node, may be assigned a uniqueaddress. As the system supports a master with multiple slaves,point-to-point ranging, as well as point-to-multipoint ranging areenabled.

Operation of the high-resolution active reflector radio frequencyranging system will now be described. A first unit (the acting master)transmits a radio signal burst asking for a ranging measurement. Asecond unit (the acting slave) determines, either by default or bydecoding a read range data packet, that it is the unit from which theacting master is requesting the ranging measurement. Following apositive determination, the acting slave measures phase and frequencydrift of the incoming carrier wave and aligns the its own oscillator, orclock, so as to achieve commonality of frequency and phase coherencewith the incoming signal. Accuracy of oscillator alignment within theslave unit can be enhanced by transmitting multiple packets. The slaveextracts phase and frequency data from each packet and averages theresults: The more packets that are received over time, the more accuratethe calculation of the phase and frequency of the incoming carrier andthe readjusting of the slave's internal clock.

For a preferred embodiment of the invention, an adaptive loop isemployed to measure the phase of random incoming packets from the masterand adjust the slave unit's oscillator so that it is phase coherent withthe master unit's oscillator. As with much of the prior art, nocontinuous wave transmission is required. In fact, the incoming RFsignal can transmit multiple packets over multiple frequencies duringdifferent periods of time. The preferred embodiment of the inventionalso incorporates a delta sigma phase lock loop, which maintains phasecoherency of the of the slave unit's oscillator with the incomingsignal, regardless of its frequency. Software onboard the slave unit isused to process incoming signal information and reconstruct it in orderto maintain phase lock of the slave unit's oscillator with that of themaster. This feature facilitates the implementation of frequencyhopping, which is instrumental in determining measurement of absolutedistances between master and slave units.

The preferred embodiment of the invention also employsthermally-insulated reference oscillators, which need be neither highlystable over time, nor highly accurate at a rated temperature. However,the thermally-insulated oscillators are very stable over short periodsof time commensurate with the periods required either by the master unitto send a burst signal and receive a burst signal in response, or for aslave unit to receive, analyze, and retransmit a signal burst. Athermally-insulated quartz crystal oscillator can be fabricated byencapsulating the oscillator within an Aerogel® insulation layer.Aerogel is an ideal insulator for the application, as it has acoefficient of expansion that is virtually identical to that of quartz.Thus, in the case of a slave unit, its thermally-insulated referenceoscillator is adjusted in frequency and phase to match thosecorresponding characteristics of the carrier wave received from themaster unit, and the signal is retransmitted to the master. This processoccurs over such a short period of time that any frequency drift in thethermally-insulated reference oscillator is negligible. Athermally-insulated reference oscillator (TIRO) has a huge advantageover an ovenized oscillator in terms of both cost and energyconsumption. For battery powered devices, ovenized oscillators arehighly impractical, as they must remain heated even when not in actualuse in order to maintain stability. A 16 MHz thermally-insulatedreference oscillator developed for the prototype high-resolution activereflector radio frequency ranging system has exhibited driftcharacteristics of less than 2.5 parts per 10 billion over a period ofone second. Using this type of reference oscillator, the system iscapable of ranging accuracies of better than 0.125 mm.

When the master unit transmits a radio frequency burst at a particularfrequency to a slave unit, the signal is received by the slave unit,mixed with at least one local oscillator signal to create an errorsignal, which is fed to a digital control system consisting of a centralprocessing unit or state machine. The output from the digital controlsystem is fed to the reference oscillator, which controls the deltasigma phase lock loop, which in turn, controls the local oscillator.Because the individual bursts may be too short to generate an accuratedetermination of phase and frequency error, several bursts may berequired to achieve optimum lock-on of the slave unit's referenceoscillator. Thus, the TIRO retains the incoming phase and frequencyinformation so that no matter on which channel the phase lock loop (PLL)is initially set, it derives its phase information from the referenceoscillator. Thus, as the TIRO sets the phase and frequency of the PLL,the TIRO also effectively sets the frequency of the slave unit'stransmitter and local oscillator.

There are two major problems associated with divide-by-integer phaselock loops. The first is that if sufficient bandwidth is allocated tothe low-pass filter for a required modulation range, there isinsufficient step resolution for both frequency generation and frequencymodulation. The second is that if smaller frequency steps are utilized,there is insufficient band width at the low-pass filter. Fractionalphase lock loops (also known as delta sigma phase lock loops) weredeveloped to solve precisely these problems. For example, in oneembodiment of the invention, the fractional PLL generates 64 clock cyclephase relations (diffs) of the local oscillator for each cycle of the 16Mhz reference oscillator. However, when a fractional PLL is used, thewave form edges of the generated signal may not directly align with thereference oscillator. This is especially problematic in a ranging systemwhere synchronicity of phase relationship between transmitted andreceived signals is essential for meaningful distance measurements. Inaddition, if burst-mode operation or frequency-hopping is envisioned, orif the local oscillator—for the sake of circuit simplicity and minimalpower consumption—is shared between transmit and receive functions, itis essential that the phase relationship between the transmitted and thereceived signal be establishable at all times. The present inventionemploys a phase relationship counter, which keeps track of thefractional time frames of the fractional phase lock loop as a functionof the reference oscillator. The phase relationship counter providesabsolute phase information for an incoming burst on any channel withinthe broadcast/receive band, thereby enabling the system to almostinstantaneously establish or reestablish the phase relationship of thelocal oscillator so that it synchronized with the reference oscillator.The phase relationship counter, coupled with the thermally-insulatedreference oscillator that ensures synchronicity of master and slavereference oscillators with negligible drift over short periods of time,allows the system to: minimize power consumption by cutting power to allbut the reference oscillator and phase-relationship counter when it isnot receiving or transmitting signals; utilize a commonvoltage-controlled local oscillator for both receive and transmitoperations; and maintain predictable phase relationships between thelocal oscillator and the received signal for both discontinuous burstsat the same frequency and bursts at different frequencies (frequencyhopping). Frequency hopping greatly enhances the usefulness of thesystem, as noisy channels can be avoided and the presence of multipathtransmissions can be detected and eliminated from ranging calculations.Frequency hopping can be used with any radio technology where adequatebandwidth is provided.

The radio transceivers used to implement the present invention employquadrature phase modulation (QPM). Like all modulation schemes, QPMconveys data by changing some aspect of a carrier signal, or the carrierwave, (usually a sinusoid) in response to a data signal. In the case ofQPM, the phase of the carrier is modulated to represent the data signal.Although the invention can be implemented by calculating the phase shiftof incoming data packets, it can also be implemented by demodulating thephase shift of the QPM data packets and using the resulting data tocalculate range.

Vernier measurement techniques can be employed to enhance the accuracyof distance calculations for the present invention. Although verniermeasurement has been used in FM radar systems for at least fifty years,those systems typically relied on the simultaneous transmission to twoor three signals at different frequencies. The present invention, on theother hand, is unique in that vernier measurement can be implementedusing randomly-selected frequencies within randomly-selected channels,which are transmitted during randomly-selected time intervals. This isbecause the phase relationship counter associated with the slave unit'sfractional phase lock loop allows the phase relationship of any receivedsignal to be established as a function of the slave reference oscillatorwhich, for relatively short periods of time, can be consideredsynchronous with the master reference oscillator. Vernier measurementsare made in the following manner: At least two signals, which are inphase at the point of transmission, are transmitted on differentfrequencies. A course measurement of distance can be made by measuringthe phase difference between the signals. Two frequencies suffice ifthey will not share a common null point over the measured distance. Fortwo-signal measurement, the bandwidth required depends on how accuratelyphase difference between the two signals can be measured. If measurementaccuracy is 3 degrees, then bandwidth can be 0.833 percent of a 400 MHzband, which is a 3.33 MHz-wide band, or two channels that are 3.33 MHzapart. If measurement accuracy is 1 degree, then bandwidth can be 0.277percent, or 1.11 MHz of the same band. Vernier ranging can be easilyimplemented on the band specified for wireless personal area network(WPAN) in North America under IEEE specification 802.15.4-2006, as itprovides for thirty channels within a bandwidth of 902-928 MHz. Ifresolution of the receiver is less than 1 wave length, phase of areceived signal can be measured. A coarse measurement provides thenumber of wavelengths from the transmitter. By calculating absolutephase of the received signals, a fraction of a wavelength can then beadded to the number of wavelengths from the transmitter for a moreaccurate calculation of range. In accordance with the present invention,it is possible to build a radio which can resolve the phase of receivedsignals down to as little as 0.1 degree. With such a radio, phasedifferences between two adjacent frequencies within a narrow band can beeasily resolved. In a band having a wavelength of 12 cm, theoreticalresolution for ranging measurements can be better than 0.005 cm.

As previously stated, two frequencies can be used for rangingcalculations up to a distance where the first null point occurs (i.e.,both frequencies once again are momentarily in phase with one another.Two radio signals at different frequencies will, at some distance fromthe source, eventually null each other out, thereby making measurementsbeyond that point ambiguous. Thus, at least three frequencies arerequired to avoid ambiguous measurements. It is particularly helpful ifthe third frequency and one of the other two frequencies do not possessa divide by n relationship. Because the ranging system of the presentinvention utilizes a fractional phase lock loop with a phaserelationship counter, random frequency hopping can be employed. Whenoperating in the 902-928 MHz band, for example, the present inventioncan randomly employ any three or more of the 30 channels over time.

A major advantage of the present invention is that it addresses ranginginaccuracies caused by multipath transmissions. Although amulti-frequency ranging system works well if transmissions are madethrough a conductor or with a laser, a radio transmission through spacegenerally results in reflections of the transmitted wave front,resulting in multipath transmission paths. As any path other than astraight line between the transmission and reception points isnecessarily of greater distance, the signal which provides the correctphase shift for accurate ranging will be accompanied by signals thathave traveled greater distances and which, therefore, display increasedamounts of phase shift. The ranging systems constructed in accordancewith the present invention transmit at least three radio signals atdifferent frequencies and compare the distance-phase relationshipbetween the different frequencies. The ranging system of the presentinvention utilizes a frequency-hopping approach to identify multipaths,select the shortest path, and calculate the distance of the shortestpath. This is uniquely accomplished by constructing a table of measuredphase and amplitude vs. frequency for at least three frequencies, whichcan be randomly selected in order both to avoid noisy channels andutilize only a small portion of available bandwidth at a given time. Ananalog-to-digital converter inputs phase-amplitude data into the tablein frequency order. This data is subjected to a Fourier transform,preferably using a computer system to perform the calculations. Theresulting beat-frequency peaks correspond to the various detected paths.The path having the lowest beat frequency is the shortest and actualdistance between the system master and slave units. Using digital signalprocessing, if an inverse fourier transform is performed on the fouriertransform data, the inverse fourier transform data can be used tocalculate changes in the phase relationships for different frequencies,and correct for distortion caused by multiple reflective paths as themaster and slave units move with respect to one another.

Vernier distance measurement and multi-path detection and correctionwork in concert. The process is performed using the following sequenceof steps. Firstly, using frequency hopping involving at leastfrequencies f1, f2 and f3, phase differences between the variousfrequency pairs (i.e., between f1 and f2, f1 and f3, and f2 and f3) aredetermined. Secondly, multipath correction is performed to eliminatemultipath data and determine the integer number of wavelengths at one ofthose frequencies that separate the master and slave unit antennas forthe shortest path. Thirdly, the system switches to a phase accumulationmode and calculates the absolute phase of each received frequency,thereby providing data for calculation of a partial wavelength that mustbe added to the integer number of wavelengths distance for an accuratemeasurement. Thus, the ranging system for the present invention provideshigh resolution range measurements with low bandwidth utilization.Although the transmission of multiple frequencies is required for theinitial distance calculation, as long as the object doesn't move morethan one-half wavelength between measurement calculations, it can betracked with a single frequency. In a gaming system, for example, theuse of a single frequency between antenna pairs once positionacquisition is achieved will greatly reduce computational overhead.

The uniqueness of the present invention is grounded in synchronizationof the reference oscillators of the master and slave units, regardlessof frequency, and in the use of thermally-insulated referenceoscillators and phase-lock loops to establish and maintain phasecoherency between master and slave units across all frequencies. Inaddition, the use of frequency hopping enables not only the randomselection of low-noise channels, but also multipath data elimination,thereby provide high-resolution measurements with minimal bandwidthrequirements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a first implementation of the radiofrequency ranging system of the present invention showing a singlemaster transceiver interacting with a single slave transceiver;

FIG. 2 is a block diagram of a second implementation of the radiofrequency ranging system of the present invention showing a singlemaster transceiver interacting with multiple slave transceivers;

FIG. 3 is a block diagram of a third implementation of the radiofrequency ranging system of the present invention showing multiplemaster/slave transceivers interacting with each other;

FIG. 4 is a block diagram of a first embodiment superheterodynetransceiver having a thermally-insulated reference oscillator and adivide-by-n phase lock loop for achieving frequency and phase coherencyof the reference oscillator with a received radio signal;

FIG. 5 is a block diagram block diagram of a second embodimentsuperheterodyne transceiver in which the phase lock loop is allowed tochange frequency only when a latch is actuated by an overflow signalfrom a phase relationship counter, an event that corresponds to a timewhen all dithered signals corresponding to channel frequency steps arein phase or when they have a predictable phase relationship;

FIG. 6 is a block diagram of a third embodiment superheterodynetransceiver having a phase accumulator of a standarddelta-sigma-modulated phase lock loop which keeps track of the phaserelationship of the base channel, a multiplier which keeps track of thephase angle offset between a selected channel and the base channel, andan adder which sums the output of the phase accumulator and themultiplier to give the phase angle of the selected channel;

FIG. 7 is a block diagram of another embodiment of a superheterodyne.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a system for low-cost radio frequencyranging that is grounded in establishing and maintaining phase andfrequency coherency of signals received by a slave unit from a masterunit and retransmitted to the master unit by the slave unit. Phase andfrequency coherency is established through the use of a phase-lock loop,and maintained through the use, on both master and slave units, ofthermally-insulated reference oscillators, which are highly stable overthe short periods of time during which communications occur between amaster unit and a slave unit. For preferred embodiments of the presentinvention, fractional phase-lock loops (also known as delta sigmaphase-lock loops having a dithered divide ratio) are employed in orderto provide both sufficient bandwidth at the low-pass filter, as well asadequate step resolution for both frequency generation and frequencymodulation. In order to keep track of the fractional time frames of thefractional phase-lock loop as a function of the reference oscillator, aphase relationship counter is employed. The phase relationship counterprovides absolute phase information for an incoming burst on any channelwithin the broadcast/receive band, thereby enabling the system to almostinstantaneously establish or reestablish the phase relationship of thelocal oscillator so that it synchronized with the reference oscillator.The system lends itself to frequency hopping operation, which permitsrandom selection of low-noise channels, as well as the elimination ofmultipath data, thereby providing high-resolution measurements withminimal bandwidth requirements.

The invention will now be described with reference to the attacheddrawing figures, which include sufficient detail so that those havingordinary skill in the art will be able to recreate the variousembodiments thereof.

Referring now to FIG. 1, a first implementation of the radio frequencyranging system 100 employs a single master transceiver M-XCVR, whichinteracts with a single slave transceiver S-XCVR;

Referring now to FIG. 2, a second implementation of the radio frequencyranging system 200 employs a single master transceiver M-XCVR1, whichinteracts with multiple slave transceivers S-XCVR-1, S-XCVR-2, . . .S-XCVR-N.

Referring now to FIG. 3, a third implementation of the radio frequencyranging system 300 employs multiple master/slave transceivers XCVR-1,XCVR-2, XCVR-3, XCVR-4, XCVR-5, . . . XCVR-N, which interact with eachother.

Referring now to FIG. 4, a first embodiment superheterodyne transceiver400 incorporates quadrature architecture (both I and Q signals) fordigital transmission and reception and a divide-by-n phase lock loop,where n is an integer. An antenna 401 is used for both receive andtransmit functions. An RF switch 402 switches between receive (RX) andtransmit (TX) modes. A transmission signal driver 403 feeds the TX poleof RF switch 402. In the receive path, a front-end image rejection mixerstage 404 generates an intermediate frequency for both I and Q signalsby mixing them with inputs from a primary voltage-controlled oscillator(VCO) 405. The intermediate frequency I and Q signals are fed to anintermediate frequency image-reject filter and amplifier stage 406, thento an analog-to-digital converter stage 407, then to a quadrature phasedecoder stage 408. The decoded digital signal is then fed to a CPU 409.After leaving the intermediate frequency filter and amplifier stage 406,the I and Q signals are also fed to a secondary image rejection mixerstage 410, which performs a secondary pass on the received data in orderto optimize it for ranging calculations. Mixer stage 410 mixes the I andQ signals with inputs from a secondary voltage-controlled oscillator411. The secondary voltage-controlled oscillator 411 is synchronized toa reference oscillator 412. A secondary phase lock loop (not shown)maintains the lock of the secondary VCO 411 on the reference oscillator412. After being passed through a low-pass filter (not shown), the I andQ signals are converted to DC signals, which are fed into a two-channelA-to-D converter 413. The resultant digital signals are fed into the CPU409. As the primary VCO 405 is used for both transmit and receivefunctions, the circuitry must be phase and frequency coherent betweentransmit and receive. The signal from the primary VCO 405 is fed to adivide-by-n prescaler 415, and then to a phase lock loop block 414,which consists of a phase detector 414A, a charge pump 414B and a lowpass filter 414C. The function of the divide-by-n prescaler 415 is toconvert the selected transmit or receive frequency to the frequency ofthe reference oscillator 412. The output from the phase lock loop block414 is fed into the primary VCO 405, thereby forming the loop. If thefirst embodiment transceiver 400 is operating is a slave mode, the CPU409 can calculate the phase difference of an incoming signal and, usinga PLL-type algorithm, lock the reference oscillator (RO) 412 on theincoming signal. The primary VCO is also locked on to RO 412 via thedivide-by-n prescaler 415 and phase lock loop block 414, so that allblocks of the transceiver 400 are phase and frequency coherent. In themaster mode, the CPU simply reads data out of block 6 (the I and Q),turn that into a phase info and use that in ranging algorithm withoutadjusting the RO. If the first embodiment transceiver 400 is operatingin a transmit, or master, mode, the frequency of primary VCO 405 isshifted to the primary transmission frequency by adjusting thedivide-by-n factor in the divide-by-in prescaler 415. As the primary VCO405 locks onto the reference oscillator 412 by means of the phase lockloop block 414, an output signal is generated that is frequency andphase coherent with the RO 412.

As heretofore explained in the Summary section, there are two majorproblems associated with divide-by-integer phase lock loops. The firstis that if sufficient bandwidth is allocated to the low-pass filter fora required modulation range, there is insufficient step resolution forboth frequency generation and frequency modulation. The second is thatif smaller frequency steps are utilized, there is insufficient bandwidth at the low-pass filter. Although use of a fractional phase lockloop solves the problems of insufficient step resolution and low-passfilter bandwidth, it creates an additional problem that waveform edgesof the generated signals may not directly align with the referenceoscillator 412. This is especially problematic in a ranging system wheresynchronicity of phase relationship between transmitted and receivedsignals is essential for meaningful distance measurements. Thesuperheterodyne transceivers of FIGS. 5, 6 and 7 address the problem ofwaveform misalignment in three different ways

Referring now to FIG. 5, a second embodiment superheterodyne transceiver500 employs a dithered divide prescaler 501 for implementing a standarddelta-sigma-modulated phase lock loop block 414. The phase lock loopblock 414 includes a phase frequency detector 414A, a charge pump 414B,and a low pass filter 414C. The output from the phase lock loop block414 is received by the primary voltage-controlled oscillator 405.Although, as is true for all phase-lock loops employing dithered divideprescalers, the waveform edges of signals generated for the multiplefrequencies corresponding to the various channels within thetransceiver's transmit/receive band are usually misaligned with thereference oscillator 412, all channel frequencies within that band will,in fact, be aligned with the reference oscillator 412 on a periodicbasis. This particular phenomenon of periodic alignment provides the keyto the establishment or reestablishment of phase synchronization betweenthe reference oscillator 412 and the local voltage-controlledoscillators 405 and 411 for this embodiment of the invention. The lengthof the period between conditions of alignment is determined by thefractional value 504 that is input as the X value for the X+Yaccumulator 505. A sum of X and Y values is provided as a recurring Yinput to accumulator 505 via an accumulate path 506 on each cycle of thereference oscillator 412. Whenever the accumulator 505 overflows, anoverflow signal is sent to the divide prescaler 501 via path 507. Thedithered divide ratio generated by the divide prescaler 501 isdetermined by the frequency of overflow signals from path 507. A phaserelationship counter 502 is provided that keeps track of clock cyclecounts emanating from the reference oscillator 412. The counter 502 isset so that it overflows at the precise time when all ditheredfrequencies are in phase with the reference oscillator 412. The overflowsignal from the phase relationship counter 502 is sent to a latch 503,which triggers changes in frequency of the voltage-controlledoscillators 405 and 411 at precisely the time when all ditheredfrequencies are in phase. Alternatively, the overflow signal from thephase relationship counter 502 can be programmed to trigger the latch503 at a time when all channel frequencies have a predictable phaserelationship other than alignment. There are two principal advantages ofthis second embodiment superheterodyne transceiver 500: the channelselection circuitry is simple, inexpensive, and operates with minimalpower consumption. A disadvantage of this embodiment is that channelswitching can be slow. The greater the number of channels in thetransmit/receive band, the slower the switching speed. It will be notedthat the output from the reference oscillator 412 is equipped with avariable phase delay 508. Although the phase signals generated by thereference oscillator 412 can be changed by controlling the voltage beingsent to it by the CPU 409, this method of phase change is relativelyslow. The addition of the variable phase delay 508 feature to thereference oscillator 412 enables minute changes in the phase of signalsgenerated by the reference oscillator 412 to be made almostinstantaneously.

Referring now to FIG. 6, a third embodiment superheterodyne transceiver600 also employs a dithered divide prescaler 501 for implementing astandard delta-sigma-modulated phase lock loop block 414. As with thesecond embodiment transceiver 500 of FIG. 5, the phase lock loop block414 includes a phase frequency detector 414A, a charge pump 414B, and alow pass filter 414C, and the output from the phase lock loop block 414is received by the primary voltage-controlled oscillator 405. Thedifferences between the transceiver 600 and the transceiver 500 of FIG.5 is that a phase accumulator 603 is employed to keep track of the phaserelationship between the base channel (channel 0) and the referenceoscillator 412, a digital multiplier 606 is employed to keep track ofthe phase angle offset between a selected channel and the base channel,and an adder 607 is employed to sum the output of the phase accumulator603 and the multiplier 606 to provide an output that represents thephase angle of the selected channel. This output from the adder 607 isprovided to the prescaler 501, which then provided an appropriatedithered divide value so that the two inputs to the phase frequencydetector 414A can be compared. As with the second embodiment, the lengthof the period between conditions of alignment is determined by afractional value 602 that is input as the X value for the X+Yaccumulator 603. A sum of X and Y values is provided as a recurring Yinput to accumulator 603 via an accumulate path 601 on each cycle of thereference oscillator 412. The accumulated Y value is also input to the Yvalue of the adder 607. It will be noted that the multiplier 606receives an X input from a phase relationship counter 604, and a Y input(0 to n) from a channel selector 605. The output from multiplier 606provides the X input for adder 607. The principle advantage of thisthird embodiment superheterodyne transceiver 600 is that channelswitching can be accomplished much more rapidly, and without waiting forthe phase relationships of all channels to align or reach a predictablephase relationship. The additional adder 607 and the digital multiplier606, in particular, do add additional complexity and power consumptionto the circuitry. Like the second embodiment transceiver 500, the thirdembodiment transceiver 600 can be equipped with a variable phase delay508 at the output of the reference oscillator 412, which enables minutechanges in the phase of signals generated by the reference oscillator412 to be made almost instantaneously.

Referring now to FIG. 7, a fourth embodiment superheterodyne transceiver700 also employs a dithered divide prescaler 501 for implementing astandard delta-sigma-modulated phase lock loop block 414. As with thesecond embodiment transceiver 500 of FIG. 5, the phase lock loop block414 includes a phase frequency detector 414A, a charge pump 414B, and alow pass filter 414C, and the output from the phase lock loop block 414is received by the primary voltage-controlled oscillator 405. The fourthembodiment transceiver 700 is designed to provide even faster channelswitching, but at the expense of increased system complexity andincreased power consumption. A phase accumulator 708 of a standarddelta-sigma-modulated phase lock loop keeps track of the phaserelationship of a base channel 0 with respect to the referenceoscillator 412. A digital multiplier 703 (which is essentially anumerically-controlled oscillator), having inputs from a channelselector with values 0 through n, and a phase relationship counter,which keeps track of each cycle of the reference oscillator 412 througha count which includes all possible phase offset combinations forchannels 0 through n in a given bandwidth, provides an quadrature outputrepresentative of the phase offset from base channel 0 for a selectedchannel to cos/sin table lookup module 705. The cos/sin table lookupmodule 705 provides a quadrature output to a digital to analog converter706, which in turn provides an analog quadrature output to an imagereject premixer 707. The image reject premixer 707 also receives aquadrature input from the primary voltage-controlled oscillator 405 thatrepresents the frequency offset of the base channel. The premixer 707functions to raise or lower the frequency of the base channel to a valuewhich corresponds to each of the channels 0 through n in the givenbandwidth. The adjusted frequency is provided as an input to a standardfront-end image rejection mixer stage 404.

The radio transceivers used to implement the present invention employquadrature phase modulation (QPM). Like all modulation schemes, QPMconveys data by changing some aspect of a carrier signal, or the carrierwave, (usually a sinusoid) in response to a data signal. In the case ofQPM, the phase of the carrier is modulated to represent the data signal.Although the invention can be implemented by calculating the phase shiftof incoming data packets, it can also be implemented by demodulating thephase-shift of the QPM data packets and using the resulting data tocalculate range. Thus, by monitoring the data decoded by block 408 it ispossible to use the phase shift data of the incoming QPM signal tonormalize the phase from block 410 and allow phase information to bedemodulated and used for ranging in normal data packets.

Although only a single embodiment of the invention has been disclosedherein, it will be obvious to those having ordinary skill in the art ofranging equipment design that changes and modifications may be madethereto without departing from the scope of the invention as hereinafterclaimed. The present invention may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. A ranging system for determining a rangingmeasurement, the ranging system comprising: a first unit that transmitsat least one signal on at least one carrier asking for the rangingmeasurement, the at least one signal including at least one read rangedata packet; a second unit that receives the at least one signal, thesecond unit including: a first oscillator used in receiving the at leastone signal and transmitting at least one second signal back to thesecond unit; a reference oscillator that controls the first oscillator,wherein the at least one read range data packet is used to achieve acommonality of frequency and phase coherence in the referenceoscillator; a phase relationship counter; and a fractional phase lockedloop, wherein the phase relationship counter keeps track of fractionaltime frames of the fractional phase locked loop as a function of thereference oscillator, wherein the phase relationship counter providesabsolute phase information for an incoming burst on any channel toestablish or reestablish a phase relationship of the local oscillator sothat the local oscillator is synchronized with the reference oscillator,wherein the first unit receives the at least one second signal anddetermines the ranging measurement between the first unit and the secondunit.
 2. The ranging system of claim 1, wherein the first unitcomprises: a first oscillator used in transmitting the at least onesignal and receiving the at least one second signal; a referenceoscillator that provides multiple frequencies and phase for the rangingmeasurement; and a phase coherency counter to switch the first unitbetween transmit and receive with a predictable phase, wherein a phasecoherency counter provides phase information relationships of the firstoscillator of the first unit.
 3. The ranging system of claim 2, whereinthe phase coherency counter of the second unit provides phaseinformation relationships of all frequencies of the first oscillator. 4.The ranging system of claim 3, wherein the phase coherency countercomprises: a single phase lock loop that combines input from the phaserelationship counter and a frequency setting of the first oscillator togenerate the predictable phase in the first oscillator.
 5. The rangingsystem of claim 1, wherein each signal included in the at least onesignal are transmitted at different times.
 6. The ranging system ofclaim 5, wherein three or more frequencies are used to determine therange measurement, wherein the at least one signal includes a firstsignal at a first frequency, a second signal at a second frequency, anda third signal at a third frequency, wherein at least one of the firstunit and the second unit hops between the first, second, and thirdfrequency to determine an absolute distance between the first unit andthe second unit.
 7. The ranging system of claim 6, wherein the three ormore frequencies are used to identify multi-path data, wherein the firstunit eliminates the multi-path data using at least one signal at adifferent frequency.
 8. The ranging system of claim 6, wherein thefirst, second, and third frequencies are selected to avoid noise and/orinterference from other devices using the same spectrum as the first,second, and third frequencies.
 9. The ranging system of claim 1, whereinthe predictable phase identifies a phase relationship at the second unitbetween the at least one signal received by the second unit and the atleast one second signal transmitted by the second unit.
 10. The rangingsystem of claim 9, wherein the at least one signal transmitted by thefirst unit and the at least one second signal transmitted by the secondunit have one of: the same frequencies, different frequencies, or someof the same frequencies.
 11. The ranging system of claim 9, wherein atleast some of the at least one signal and the at least one second signalcan be transmitted at random times.
 12. The ranging system of claim 1,wherein a path is selected and an inverse fourier transform is performedon data for the selected path, wherein changes in phase relationshipsfor the frequencies are determined, wherein distortion caused bymultiple reflective paths as the first unit and the second unit movewith respect to each other are corrected.
 13. A method for determining aranging measurement, the method comprising: transmitting at least onesignal from a first unit to a second unit, the at least one signalincluding read range data packets; extracting frequencies and phaseinformation of the at least one signal from the read range data packets,wherein the at least one signal includes at three or more frequencies;setting a thermally insulated reference oscillator to match thefrequencies and phase information of the at least one signal; generatingat least one second signal having a predictable phase using thereference oscillator; transmitting the at least one second signal backto the first unit from the second unit; determining the rangingmeasurement between the first unit and the second unit by a differencebetween the predictable phase of the at least one second signal at thesecond unit and a measured phase of the at least one second signal atthe first unit for multiple paths; and wherein the ranging measurementis determined by converting the at least one second signal from afrequency domain to a time domain to determine the ranging measurementsof the multiple paths.
 14. The method of claim 13, wherein transmittingat least one signal further comprises transmitting a first signal at afirst frequency, a second signal at a second frequency, and a thirdsignal at a third frequency, wherein the first, second, and thirdsignals are selected to determine an absolute distance of multiplewavelengths.
 15. The method of claim 13, wherein multi-path data isidentified by transmitting at least three signals at differentfrequencies.
 16. The method of claim 13, wherein transmitting at leastone signal further comprises hopping between multiple frequencies,wherein each of the at least one signals are transmitted at differenttimes and wherein each of the at least one second signals aretransmitted at different times.
 17. The method of claim 13, furthercomprising using the reference oscillator to provide predictable phaserelationships between all frequencies of a first oscillator included inthe second unit.